1. Field of the Invention
The invention relates generally to the recovery of heat from furnaces and, more particularly, the recovery of heat from fuel fired furnaces utilizing cassette regenerators.
2. Description of the Prior Art
Most commercial glass is produced in high temperature air/fuel furnaces where solid raw materials are melted, reacted to form stabilized silicates and degassed of entrained gases to allow downstream forming of a homogeneous product. Energy input to the furnace in the form of natural gas or oil firing and electrical resistance heating (electric boosting) melts the raw materials, provides heat of reaction and raises the molten temperature while decreasing viscosity to allow for proper degassing of the glass. The vast majority of these furnaces use air to support combustion. Different furnace designs have evolved in each segment of the glass industry which are specifically tailored to the particular demands of the end use product. Examples of traditional furnace designs include the regenerative melter, recuperative melter, all-electric melter and direct fired unit melter.
By far, the dominant furnace design for the glass industry is the regenerative melter. A typical regenerative melter includes at least two burners, two regenerators, a flow reversal system and associated controls. Paired sets of burners are located on opposed sides of the furnace or are end port fired where both systems are on the same wall of the furnace. A heat regenerator communicates with each burner. The burners and regenerators are closely coupled by a length of refractory lined duct to suit the space available on site. When the first burner of a pair fires, using combustion air fed to the base of its regenerator, the second burner of the pair acts as an exhaust port drawing off waste gas, thereby heating the regenerator for the second burner. When this heated regenerator is sufficiently charged, the reversal system operates to reverse the firing system. The second burner of the pair fires to heat the furnace and the first burner, in time, acts as an exhaust port, thereby heating the regenerator for the first burner. The combustion air is then directed through the hot regenerator of the second burner to preheat the air prior to combustion. After a period of time, the flow of exhaust gases and combustion air through the regenerators is again reversed to maintain heating of the combustion air.
These regenerators typically take the form of latticed brick work or "checkers" through which the combustion air passes on its way to the burner to preheat the combustion air and through which the exhaust gases from the furnace pass on their way to the stack. The exhaust gases transfer their sensible heat to the regenerator bricks as they pass through. On the reverse cycle, clean combustion air brought in at ambient temperature is passed through the previously heated regenerator of the firing burner and thus picks up sensible heat from the bricks. In this way, the regenerator preheats the air prior to combustion.
In an alternative traditional furnace design, recuperative heat exchangers, rather than regenerators, are used to preheat the combustion air. Recuperative heat exchangers differ from regenerative heat exchangers in that the exhaust gases and combustion air flow through different piping systems and do not mix. The recuperator acts as a simple indirect heat exchanger. Heat from the exhaust gases flowing through one conduit is transferred to combustion air flowing through another conduit.
As an alternative to conventional air/fuel furnaces, oxy-fuel fired furnaces have been developed. In an oxy-fuel fired furnace, oxygen gas instead of air is used to support combustion. Unlike conventional air/fuel furnaces, the oxygen is not preheated prior to being mixed with the fuel such as natural gas or oil. Therefore, no regenerators or recuperators are typically associated with oxy-fuel fired furnaces. Oxy-fuel fired furnaces offer some advantages over typical air/fuel furnaces, such as generally lower NO.sub.x concentrations. However, oxy-fuel furnaces, as a general rule, are more expensive to operate since the oxygen must be purchased for use in the furnace.
Glass manufacturing, generally speaking, is a high temperature, energy intensive operation where approximately 65-70% of the total energy is consumed in the melting process. Typical air/fuel fired glass melting furnaces have about 30% of their total input energy lost through the exhaust stack. Recently, there have been attempts to utilize the sensible heat in the exhaust from conventional air-fuel fired regenerative furnaces to perform useful work, such as supplying hot gases to run a turbine. In the initial attempts to use the exhaust gases from regenerative melting furnaces to run a turbine, the hot exhaust gases were cooled and supplied directly to the turbine. However, use of the exhaust gases to directly turn the turbine resulted in considerable erosion of the turbine and turbine blades due to the high amounts of particulate matter and corrosives present in the exhaust gases. Therefore, heating methods using recuperator-type heat exchangers were developed to prevent damage to the turbine blades.
One such method is disclosed in U.S. Pat. No. 4,528,012 to Sturgill. In the Sturgill patent, a typical regenerative glass melting furnace is fired by preheated air/fuel burners. Incoming combustion air is heated in a checker-type regenerator. Exhaust gas exits the opposite side of the furnace to give up its heat to another checker-type regenerator. Exhaust gas leaves the second regenerator and is directed to a recuperator-type heat exchanger. The exhaust stream flows through the recuperator and out an exhaust stack. Ambient air is fed into a filter and then to a compressor attached to a turbine. The incoming air is compressed and is fed through different piping in the recuperator where it is heated by the exhaust gas from the furnace. The compressed, preheated air is then fed to a turbine which rotates a shaft to run the compressor. The turbine shaft may also be connected to a generator to generate electrical power. The exhaust from the turbine becomes a source of preheated combustion air that is added to the checker-type regenerator. The exhaust gases which pass through the recuperator do not come in direct contact with the ambient air. While the recuperative heat recovery system disclosed in the Sturgill patent permits beneficial use of the exhaust gas, i.e., to turn the turbine, the costs involved in constructing and maintaining such recuperators adversely impacts upon the benefits derived from the use of the exhaust gas. Further, these recuperators are prone to clogging and therefore must be cleaned on a regular basis to prevent significant losses in thermal conductivity.
To date, little effort has been made to utilize the heat from the exhaust gas of high temperature furnaces, such as oxy-fuel furnaces, for alternative purposes. Use of standard regenerators in oxy-fuel systems is not advisable due to the large costs involved in construction and maintenance. Further, standard regenerators do not typically reduce the particulate concentration to an acceptable level for use in turbines. With respect to standard recuperators, the costs involved in constructing and maintaining such recuperators are prohibitive. Further, standard recuperators are prone to clogging and are difficult to clean and maintain.
Therefore, it is an object of the invention to provide a high temperature furnace system with a simple replaceable regenerator system in which the hot exhaust gases from the furnace can be used to power a turbine without the need for a complex recuperation system. It is a further object of the invention to provide an oxy-fuel furnace system in which the exhaust gas is used to run a turbine for powering an oxygen generator. The oxygen so generated is then used in the oxy-fuel furnace. It is also an object of the invention to provide a replaceable cassette regenerator heat exchanger system to recover waste heat from an oxy-fuel furnace system to preheat air for use in a turbine.